Equation of State w=1: Why Not Considered in Cosmology?

[Demystifier]

A free scalar field with Lagrangian density
$${\cal L}=\frac{1}{2}g^{\mu\nu}\partial_{\mu}\phi\partial_{\nu}\phi$$
has the energy momentum tensor with the equation of state $p=\rho$, i.e. $w=1$. The Lagrangian density above is a very natural Lagrangian, yet the equation of state $w=1$ seems not to be considered in cosmology. Why is it not considered?

 

[bapowell]

I believe $w=1$ is referred to as a "stiff fluid" in the literature. It's been considered, but I don't know how extensively or in what contexts.

 

[kimbyd]

A free scalar field with Lagrangian density
$${\cal L}=\frac{1}{2}g^{\mu\nu}\partial_{\mu}\phi\partial_{\nu}\phi$$
has the energy momentum tensor with the equation of state $p=\rho$, i.e. $w=1$. The Lagrangian density above is a very natural Lagrangian, yet the the equation of state $w=1$ seems not to be considered in cosmology. Why is it not considered?

What observational puzzles would this solve? Such a fluid would dilute extremely rapidly, with density scaling as $a^{-6}$.

 

[Demystifier]

I believe $w=1$ is referred to as a "stiff fluid" in the literature. It's been considered, but I don't know how extensively or in what contexts.

Googling "stiff fluid" gave me interesting results such as
https://arxiv.org/abs/1412.0743
https://arxiv.org/abs/1006.4166
Thanks!

 

[jerromyjon]

What observational puzzles would this solve?

Certainly not the matter/antimatter imbalance... I think that is the most important puzzle of way back when. His following link discusses elemental distribution, though.

https://arxiv.org/abs/1006.4166

"We calculate numerically the effect of such a stiff fluid on the primordial element abundances..."

Let me know if you find anything on the matter abundances, please!